The present invention relates to a papermaking felt (hereinafter also called “felt”) used for squeezing water from inside a wet paper web, onto which it is stacked, by a pair of rotating rolls or by a roll and a shoe of a papermaking machine. More particularly, the present invention relates to a papermaking felt for improving the capability to squeeze water from the wet paper web during the period including from the initial warming-up period to the top speed operation of papermaking machines at which constant production is possible.
Conventionally, papermaking machines in the papermaking process are generally equipped with a wire part, a press part and a dryer part to dewater wet paper webs. These parts are provided in the direction in which the wet paper web is transferred in the order of: wire part, press part and dryer part. The wet paper web is dewatered and, at the same time, transferred while being passed from one papermaking equipment to the next provided in the wire part, press part and dryer part, respectively, to be finally dried in the dryer part.
Papermaking equipment for dewatering is provided corresponding to each of these parts. The press device provided in the press part comprises a plurality of press devices arranged in series in the direction in which the wet paper web is transported.
Each press device comprises an endless felt or an open-ended felt that has been made into an endless felt by connecting it in the papermaking machine and, as a press, a pair of rolls (i.e., a roll press) or a roll and a shoe (i.e., a shoe press), which are provided so as to face each other and pinch therebetween one part of the felt, respectively, from above and below; wherein pressure is applied on a wet paper web, which is transported by a felt traveling at substantially a same speed and in the same direction, together with the felt by one roll and the other roll or between the roll and the shoe, whereby the moisture in a wet paper web is squeezed out and is being continuously absorbed by the felt.
Moreover, among these types of papermaking machines there are some that have a roll press mechanism, provided in the press device of the press part, for pressing while pinching, between one roll and the other roll, one part of the felt(s) holding the wet paper web (therebetween), while others have a shoe press mechanism, provided in the press device of the press part, for pressing while pinching, between the roll and the shoe, one part of the felt(s) holding the wet paper web (therebetween).
The felt is made from a base material and (a) batt layer(s); the batt layer(s) is (are) provided both on the wet paper web carrying-side and on the press roll-side of the base material or only on the wet paper web carrying side. The batt layer is made by intertwiningly integrating batt fibers with the base material by needle punching. The felt basically has the functions of squeezing water from the wet paper web (water squeezing capability), of increasing the smoothness of the wet paper web and of transferring the wet paper web.
Above all, what is considered to be important in a felt is the function of dewatering the wet paper web, the ability to maintain compressibility and water permeability, resulting from a suitable free space volume in the felt, for discharging water that has moved from the wet paper web to the felt due to passing the pressure between the pair of rolls or the roll and the shoe, to the outside of the felt system.
The suitable free space volume is the free space volume during constant speed operation of a papermaking machine. From the viewpoint of productivity, it is important that the operating speed stabilizes rapidly; the time until this happens is called the initial warming-up period. The initial warming-up period differs according to the operating conditions of the papermaking machine; however, in general one to two days, at most five days, are required. In particular, with the no-draw straightthrough type of wet paper transfer method, of which the Tandem-Nipco Flex papermaking machine is a representative example, it is important to shorten the initial warming-up period and to increase operating speed.
Various conventional felts have been developed from this point of view. For example, after a felt has been prepared, a commonly known technique is to apply pressure to make the felt thinner during the following processes and to increase the density. There are also cases in which the felt is brought into contact with a roll that has been heated by a heating medium in order to increase the effect of the pressing. The operating mechanism is to reduce the free space volume in the felt and to facilitate the transfer of the pressing force received in the press part to the wet paper web.
In Patent document 1 (JP-T-2005-524002), a compacting method is described in which the felt surface is polished after it has been treated with a polymer substance. A felt of this structure is compacted from the beginning; therefore, it leads to the shortening of the initial warming-up period of the papermaking machine.
Nevertheless, even though a papermaking felt using the polyurethane, polycarbonate urethane, polyacrylate, acryl resin, epoxy resin, phenol resin or mixed polymers thereof according to Patent document 1 can be compacted due to the adhesive force and the coagulating force of the polymers, stiffness is given to the felt as a whole. When the stiffness becomes too great, the compression/recovery behavior under the press is suppressed and sufficient wet paper web water squeezing performance cannot be obtained; further, when the felt is placed in a papermaking machine, together with the difficult operation of manually inserting the felt into the narrow space between the rolls, there is also a problem with regard to the easiness of loading the felt.
In Patent document 2 (JP-A-02-127585), a manufacturing method for coating a foam resin onto a felt surface and for drying and curing the same is described. In a felt of this structure, the felt surface, which due to the foam resin has a porous contact region, removes the water from the wet paper web.
Nevertheless, in the felt described in Patent document 2, the porous part can receive the moisture that is squeezed from the wet paper web when it is new; however, under the direct impact of the repeated pressure from the press roll, the felt, including the porous part, is gradually compacted. There is the problem that the water squeezing capability deteriorates because, when the foam resin layer is compacted, the water permeability decreases, and when it accumulates dirt from the wet paper web, it becomes impossible to receive the moisture from the wet paper web.
Patent document 3 (JP-A-2005-146443), which employs the same type of foam resin, proposes a production method, wherein a layer (wall structure) is provided by a foam gel inside the wet paper web contact layer on the felt base material. In a felt of this structure, which has good pressure dispersion, base fabric marking is prevented and the surface smoothness of the wet paper web is improved.
Nevertheless, in the felt described in Patent document 3, even though the gel foam layer is not in direct contact with the press roll, there is the same problem as in Patent document 2.
With the felt described in Patent document 4 (JP-A-56-53297), it can be expected that the initial warming-up period is shortened due to the hydrophilic properties of the sodium acrylate-acrylamide copolymer fibers.
Nevertheless, with the felt described in Patent document 4, there is the problem that the ability to maintain the water squeezing capability deteriorates, because the durability of the sodium acrylate-acrylamide copolymer fibers is low. There is further the problem that the fibers of low durability are shed from the felt and get attached to the paper, which is a hindrance during printing.